Known in the art is a stator of a cryogenic electric machine with a superconducting field winding (cf. a paper by Dombrovsky V. V. et at "Voprosy proektirovanija kriogennih turbogeneratorov bolshoy moszhnosti" in a book "Visokoispolzovannie turbo- i gidrogeneratori s neposredstvennium ohlazhdeniem", Nauka Publishers, 1971, pp. 3-6), which stator comprises a housing accommodating a magnetic circuit and a winding. The magnetic circuit is a cylindrical laminated core made of electrical steel, the winding being fixed on the inner surface of the core, facing the rotor of a machine. The core may have both toothed and toothless construction. Direct liquid cooling of the core is provided through axial passages made therein to receive tubes filled with cooling liquid circulating within a closed system including an external cooler. Alongside with this direct liquid cooling of a stator winding is used.
However, the possibilities of liquid cooling of the core in the above stator have certain limitations. It is evident that the efficiency of cooling is increased with the increase in the number of axial passages provided in the stator core and used for circulation of a coolant, as well as with the increase in the area of cross-section of these passages. However, the number of said passages and the area of cross-section thereof are limited by the condition of providing sufficient mechanical strength of the core and the required area of cross-section of the magnetic circuit (active steel) which could not be decreased arbitrarily. This limits the possibilities of direct cooling of the core and stator as a whole. Besides it should be noted that direct cooling of the stator winding is known by a complicated way of feeding the coolant to current-carrying stator elements.
The problem of cooling becomes more acute for high-power turbogenerators, as well as for electric machines having a toothless core wherein its direct water cooling is complicated because of the need to use rather thin conductors of the winding thus decreasing the total efficiency of heat removal from a stator.
It should be also pointed out that the above stator has a comparatively large mass and overall dimensions due to the use of a steel laminated core as a magnetic circuit, while in cryogenic machines there is no need to use a large core, which is explained by specific peculiarities of these machines.
It is evident to those skilled in the art that in a rotor with superconducting field winding there are no limitations to the magnitude of magnetizing current. In this case, a magnetic excitation field of required intensity may be formed irrespective of the size of an air (nonmagnetic) gap in the machine and, which is of most importance, irrespective of the dimensions of a stator magnetic circuit and physical properties of the material which a magnetic circuit is made of. Moreover, said field may be formed in a stator even without a magnetic core which is accompanied, however, by extreme complication of the problem of retaining a stator winding and by penetration of the excitation field outside the machine so that a considerable part thereof becomes passive forming so called stray field.
Consequently, in cryogenic electric machines the role of a stator magnetic core is essentially different from its traditional role in conventional machines. In cryogenic machines a core takes upon itself dynamic forces from the winding and shields the space surrounding the machine from the excitation field, i.e. serves for retaining the winding and for localization of magnetic field within the limits of an active zone of the machine preventing it from leaking outside thereof. In conventional machines the main role of a stator core is only to enhance the magnetic excitation flux.
That is why in conventional machines a stator core is made sufficiently large so that induction therein would not exceed those values at which permiability of core steel starts to decrease. There is no need to enhance excitation flux in cryogenic machines while localization of the magnetic field in the active zone of a machine, shielding of the surrounding space and retaining of the winding may be achieved using a core of considerably smaller dimensions which provide an essential increase in induction thereof and a several times decrease in permeability of core steel. However, as mentioned above, the decrease in permeability of a stator core is of no importance in cryogenic machines, since it is possible to provide current rise in the superconducting field winding without extra expenditures of energy. Practically it is sufficient for cryogenic machines that the value of permeability of the stator magnetic circuit be equal to several tens of units, instead of hundreds thereof in a conventional machine at the highest saturation.
Thus, application of a magnetic circuit in the form of a comparatively massive steel core in the aforementioned stator is not functionally justified, resulting in the increase in stator mass and overall dimensions thereof. Application of an all steel magnetic circuit in a stator of a cryogenic machine is not reasonable because the high value of the magnetic flux results in strong saturation of the steel, making its use as a magnetic material inefficient and uneconomic.